1. Field of the Invention
This invention relates to piston-type vacuum pumps. More particularly, the present invention relates to an improved piston-type vacuum pump with a zero or negative clearance valve for allowing more complete evacuation of the vacuum chamber, and hence more efficient pumping.
2. State of the Art
Piston-type vacuum pumps are well known. Such pumps typically comprise a reciprocating piston disposed within a cylinder, the cylinder having inlet and outlet ports or valves. Uncompressed air is drawn into an evacuated cylinder through an inlet port, then compressed and forced to flow out through an outlet valve by the forward motion of the piston. As the piston retracts, the outlet valve closes, allowing the retraction of the piston to create a vacuum within the cylinder. When the piston reaches or passes the inlet port, the vacuum formed within the cylinder draws gas into the cylinder, and the process repeats itself.
However, one drawback of typical piston-type vacuum pumps is that the cylinder never completely evacuates at the end of the compression cycle. When the piston extends to the forward position, clearances between component parts allow a small amount of gas to remain in the cylinder. For example, if the pump utilizes a pressure activated outlet valve, such as a spring-loaded valve, some residual pressure will normally remain in the cylinder when the valve closes because a minimum pressure is required to open the valve in the first place due to the spring-biased configuration. Additionally, the clearance between the end of the piston and the inside of the end of the cylinder or outlet valve normally defines a volume of space in which some air will naturally remain.
In order to create a piston-type vacuum pump with increased vacuum pressure capability, it would be desirable to have a piston pump configured to allow no residual gas to remain in the cylinder after.compression. It would also be desirable to have a piston-type vacuum pump which requires very low power to operate.